How are assets/public directories accessed in web servers?

Whenever you reference something in your public or assets directory via link/script tags, are you only downloading what you reference?

For example, say I have an enormous amount of images in my public directory. Only images that are referenced on that particular page are downloaded, right?

Taking this further, say your stylesheet references a lot of images, and it serves every single page on the your website. So are all of those images downloaded?

You see, I'm probably lacking some fundamental background on this as I'm pretty new to web dev, and I don't have much experience with nginx/apache or the like. Please explain!